Microlithographic projection exposure apparatus are used in order to transfer structures, which are contained in a mask, onto a photoresist or another photosensitive layer. Important optical components of a projection exposure apparatus include a light source, the illumination system which conditions the projection light generated by the light source and directs it onto the mask, and the projection objective which images the mask illuminated by the illumination system onto the photosensitive layer.
When the wavelength of the projection light is shorter, commensurately smaller structures can be defined on the photosensitive layer with the aid of the projection exposure apparatus. It is believed that the next generation of projection exposure apparatus will use projection light in the extreme ultraviolet spectral range (EUV), the wavelength of which is at 13.5 nm. Such projection exposure apparatus are often referred to as EUV projection exposure apparatus.
However, it is believed that there may be no optical materials which have a sufficiently high transmissivity for such short wavelengths. EUV projection exposure apparatus therefore exclusively contain reflective optical elements, and the mask therefore also contains a pattern of reflective structures. The provision of mirrors for EUV projection exposure apparatus, however, represents a great technological challenge. Coatings which are suitable for EUV light, and are applied onto a mirror substrate, often include more than 30 or 40 double layers with a thickness of only a few nanometers, which are vapour deposited over one another in technologically elaborate processes. Even with such elaborately constructed coatings, the reflectivity of the mirrors for EUV light is usually scarcely more than 70%, this being only for light which strikes the reflective coating perpendicularly or with incidence angles of a few degrees.
The comparatively low reflectivity of the mirrors means that attempts have been made to use as few mirrors as possible when developing projection exposure apparatus, since each mirror involves light losses and in the end reduces the throughput of the projection exposure apparatus. The relatively low reflectivity of the mirrors, however, also entails thermal problems since the fraction of the energetic EUV light not reflected by the coating is often absorbed and can lead to a temperature increase of the mirrors. The heat thereby generated is to be dissipated essentially via thermal conduction through the mirror substrate, since the projection exposure apparatus have to be operated in a vacuum owing to the high absorption of EUV light by gases.
Materials which have a thermal expansion coefficient that is very small are often used for the mirror substrates to reduce temperature gradients occurring in the mirror substrates that could lead to undesired deformation of the mirrors. Such glass-based materials are marketed, for example, by Schott under the brand Zerodur® and by Corning under the brand ULE®. By additional measures, thermal deformations which are caused by absorption of EUV light can be kept small, or at least their effects on the optical properties of the projection objective are kept within tolerable limits.
As an example, U.S. Pat. No. 7,477,355 B2 proposes to heat the mirrors using an additional heating mechanism so that their substrate material is at a temperature at which the thermal expansion coefficient is zero or at least a minimal. It is reported that temperature variations during operation of the apparatus then have no effect, or only little effect, on the imaging properties of the mirror.
U.S. Pat. No. 7,557,902 B2 describes a projection objective in which two mirrors contain materials whose thermal expansion coefficient increases with increasing temperature for one of the two mirrors and decreases with increasing temperature for the other mirror. It is disclosed that, with suitable selection of the mirrors, the effect which can be achieved in this way is that although the two mirrors deform significantly in the event of a temperature change, the optical effects of these deformations nevertheless substantially cancel each other out.